Digital Micromirror Array (DMA) technology comprises an array of individual moveable micromirrors over memory cells of a CMOS static RAM, for example the digital micromirror device (DMD) produced by Texas instruments. Electrostatic forces based on the data in each memory cell tilt each individual micromirror on torsional hinges, typically by and angle of .+-.10 degrees from the plane of the overall DMA. The individual micromirrors are switched between the "on" and "off" state, while DMA components other than the mirrors can be found in a "flat" orientation with no tilt.
In DMA-based optical systems and devices (e.g., light projectors) the .+-.10 degrees (total 20.degree.) tilt of the micromirrors is the factor limiting the maximum aperture of the objective lens projecting the light reflected from the DMA surface and imaging it on a screen. According to the prior art shown in FIG. 1, incident light 100 is passed through an illuminating lens 103 and focused on a digital micromirror array (DMA) 102. It is then reflected depending on the positions of the micromirrors.
Normally, desired light reflected from micromirrors in the "on" position, i.e., the "on" reflected light cone 104, is collected by an objective lens 106. The maximum vertex (and consequent width) of the "on" reflected light cone 104, corresponds to the objective lens 106 aperture, which is typically F/2.8. It is noted that the F/# is an optical term known in the art and defined as the focal length of the lens divided by the diameter of the lens. The "speed" of a given lens refers to the F/#. In general, the "faster" the lens, the lower the F/#.
Similarly, undesired light reflected away from the objective lens 106 from micromirrors in the "off" position, forms the "off" reflected light cone 110. Undesired light reflected by "flat" components of the DMA (i.e., those components of the DMA other than the "on" and "off" micromirrors) form the "flat" reflected light cone 108. These undesired "flat" and "off" light cones 108 and 110 are diverted away from objective lens 106 to a light absorber 112. FIG. 1 shows the illuminating light 100 and all possible reflected light cones (104, 110, 108) based on the mirrors in the on and off positions, as well as contributed by reflections from flat components of the DMA.
Prior art imaging systems such as illustrated in FIG. 1 typically are optimized for a standard 10.degree. tilt of the micromirror by using an objective lens 106 with an aperture of F/2.8 since at this aperture the objective lens 106 collects only the "on" reflected light cone 104 and there is no overlap with undesired light in either the "flat" or "off" light cones. It is to be noted that FIG. 1 is a schematic representation insofar as the light cone vertex angles are illustrated to be larger than 20.degree. simply to facilitate drawing clarity and emphasis. In reality, these vertex angles are in fact approximately 20.degree.. Each light cone then emerges tilted from the adjacent light cone by approximately 20.degree. so that the light cones emanate from DMA 102 as closely as possible to one another without overlap, as shown. Thus, there is a 20.degree. rotation of the "flat" light cone relative to its adjacent "on" light cone, and further of the "off" light cone relative to its adjacent "flat" light cone.
To understand this fully, it is important to note that for a given degree of tilt of one mirror relative to another, the reflected light beams from a common incident light source will differ in angle from one another by twice this degree of tilt (i.e., the reflection angle is doubled), as can be seen by considering basic optical principles of light reflection. The 20.degree. vertex of each light cone results from choosing illuminating lens 103 so as to focus the incident light 100 on DMA 102 with a similar 20.degree. vertex; the 20.degree. tilting of each light cone relative to its adjacent cone results from the 10.degree. difference between the "on" micromirrors and the "flat" DMA components, and between the "flat" DMA components and the "off" micromirrors, which difference becomes doubled upon the reflection of the incident light 100.
Therefore, typical prior art DMA-based systems, with a representative 10.degree. micromirror tilt producing a 20.degree. angular reflection between adjacent light cones, are limited to using an objective lens aperture no better than F/2.8. Using a faster lens with a larger aperture in combination with using a wider vertex angle for all of the reflected light cones would produce overlap between the light cones, and thereby degrade the image produced since the lens would capture light from the undesired light cones and the resultant image would include artifacts from the undesired overlapping light cones. The prior art is thus lacking a means of benefiting from the use of an objective lens faster and wider than that which corresponds to no overlap between reflected light cones, without introducing image degrading artifacts that essentially negate the improvements normally gained by increasing the lens aperture in a projection system.
More generally, a micromirror 102 with any given degree of tilt imposes a corresponding limiting objective lens 106 aperture based on the requirement that the undesired "flat" 108 and "off" 110 reflected light cones do not overlap with the desired "on" reflected light cone 104, since the latter fills the area collected by objective lens 106.